1. Field of the Invention
The present invention relates to a planar optical waveguide device having a function of light splitting, wavelength division multiplexing/demultiplexing, level control, or switching for use in the optical communication field or the like.
2. Description of the Related Art
A planar optical waveguide is composed of a substrate, a lower cladding layer formed on the substrate, a core formed on the lower cladding layer, and an upper cladding layer formed on the lower cladding layer so as to cover the core. The core has a refractive index higher than that of each of the upper and lower cladding layers, and light is confined in the core to propagate therein. In fabricating the planar optical waveguide, the process temperature for formation of each cladding layer and a core layer is in the range of 300° C. (CVD) to 1600° C. (flame hydrolysis deposition). Thus, the difference between the process temperature and room temperature is very large.
In general, the substrate is formed of silicon(Si) or silica, each cladding layer is formed of silica glass doped with phosphorus (P) or silica glass doped with boron (B) and phosphorus (P), for example, and the core is formed of silica glass doped with germanium (Ge) and phosphorus (P), for example. Since the material of the substrate is different from that of each cladding layer, the coefficient of thermal expansion of the substrate is also different from that of each cladding layer. Accordingly, when the fabrication process for the planar optical waveguide is finished to cool it near to room temperature, stress due to the difference in coefficient of thermal expansion between the substrate and each cladding layer is generated in each cladding layer.
For example, in the case that the substrate is formed of silicon and each cladding layer is formed of silica glass doped with phosphorus (P), compressive stress is generated in each cladding layer because the coefficient of thermal expansion of the Si substrate is larger than that of each cladding layer. Conversely, in the case that the substrate is formed of silica and each cladding layer is formed of silica glass doped with phosphorus (P), tensile stress is generated in each cladding layer because the coefficient of thermal expansion of the silica substrate is smaller than that of each cladding layer.
When stress is generated in each cladding layer of the optical waveguide, the birefringence of the core is caused by this stress. That is, the refractive index of the core becomes different according to the polarization direction of light propagating in the core. As a result, the propagation constant and guide wavelength of the optical waveguide becomes different according to the polarization direction. When a normal single-mode optical fiber is connected to such an optical waveguide device for use, the fundamental characteristics of the optical device such as insertion loss vary according to the polarization direction because the polarization direction in the optical fiber is not constant.
Such a loss dependent on the polarization direction is referred to as polarization-dependent loss (PDL). In a WDM optical communication system using a normal single-mode optical fiber, it is strongly demanded to reduce the PDL. A technique of forming stress relief grooves for relieving the stress generated in each cladding layer on the opposite sides of the core so as to extend along the core is disclosed in Japanese Patent Laid-open No. Sho 63-43105. Although the stress generated in each cladding layer can be relieved by the stress relief grooves, there are many limits to the composition and dimensions of each cladding layer, and it is difficult for the stress relief grooves to completely cancel the compressive stress applied to the core.
Thus, in the optical waveguide having the stress relief grooves described in the above publication, it is difficult to completely relieve the stress applied to the core. As a result, the birefringence due to the stress in each cladding layer is left in the core to cause an increase in polarization dependence of the optical waveguide, so that the polarization-dependent loss (PDL) is generated to cause the insertion loss of the device. In a conventional optical waveguide device whose core has the birefringence due to the stress, the output level changes with a change in polarization direction of incident light, so that this optical waveguide device cannot be applied to an optical communication system using a normal single-mode optical fiber.